In a mixing bowl, combine soy sauce, olive oil, red wine vinegar, garlic, oregano, and black pepper.
Place the flank steak in a resealable plastic bag and pour the marinade over it. Seal the bag and refrigerate for at least 2 hours, or overnight for best results.
Preheat your grill to medium-high heat.
Remove the steak from the marinade and discard the marinade. Grill the steak for about 5 minutes per side, or until it reaches your desired level of doneness.
Let the steak rest for 5 minutes before slicing against the grain. Serve and enjoy!